Japanese Patent Application Laid-Open No. 2009-159727 discloses a power factor correct device having two step-up switching converters. In Japanese Patent Application Laid-Open No. 2009-159727, there are employed modes in which periods of operations of two step-up switching converters are different from each other, while switching the modes depending on an output current of the power factor correct device. In more detail, six modes are employed as these modes. In particular, the mode 0 is a mode in which the two step-up switching converters are made to operate continuously. The mode 1 is a mode in which operation/suspension of the two step-up switching converters is repeated such that parts of the operation periods of the two step-up switching converters are temporally overlapped. The mode 2 is a mode in which suspension periods of the step-up switching converters are longer than that in the mode 1. The mode 3 is a mode in which the suspension periods are still longer and the operations of the two step-up switching converters are not overlapped. The mode 4 is a mode in which one of the step-up switching converters is suspended, and the operation/suspension of the other step-up switching converter is repeated. The mode 5 is a mode in which the suspension period of the other switching converter is longer than that in the mode 4.
In Japanese Patent Application Laid-Open No. 2009-159727, when a load current is low and currents flowing through switching elements of the step-up switching converters are low, the mode 4 or the mode 5 is employed. By this, a power source efficiency is improved.
In addition, as technology related to the present invention, Japanese Patent Application Laid-Open No. 2008-193818 and Mamoru Kitamura, “Creating a 1.5 kW Low-Noise Power Supply with High Harmonic Suppression—The R2A20112 Critical Conduction Mode/Interleaving PFC IC”, Transistor Gijutsu, May 2008 Issue, CQ Publishing Co., Ltd, August 2008, pp. 176-184. are disclosed.